#include <iostream>
#include <algorithm>
using namespace std;
int main() {
    int m,s,c,result;
    cin >> m >> s >> c;
    int a[207];
    int dis[206];
    for(int i=0;i<c;i++){
        cin>>a[i];
    }
    if(m>c){
        cout<<c<<endl;
        return 0;
    }
    sort(a,a+c);
    result = a[c-1]-a[0]+1;
    for(int i=1;i<c;i++){
        dis[i-1] = a[i]-a[i-1]-1;
    }
    sort(dis,dis+c-1);
    for(int i=0;i<m-1;i++){
        result-=dis[c-2-i];
    }
    cout<<result<<endl;
    return 0;
}